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| United States Patent |
5,554,436
|
|
Katayama
, et al.
|
September 10, 1996
|
Magneto-optical recording medium
Abstract
A magneto-optical recording medium is composed of first through fourth
magnetic layers laminated in this order in such a way that an exchange
coupling exists between any adjacent layers of the first through fourth
magnetic layers so that a magnetization direction can be copied, wherein
the first magnetic layer is a memory layer, the second magnetic layer is a
writing layer, the third magnetic layer is a switching layer, and the
fourth magnetic layer is an initializing layer. Further, respective Curie
temperatures Tci satisfy an inequation Tc.sub.3 <Tc.sub.1 <Tc.sub.2,
Tc.sub.4 (i=1, 2, 3, or 4 indicating an ordinal number of the magnetic
layers); and the second magnetic layer exhibits an in-plane magnetization
at room temperature, whereas, exhibits a perpendicular magnetization above
a predetermined temperature set between the room temperature and Tc.sub.3
temperature. With this arrangement, after information is recorded on the
memory layer based on the magnetization direction, the interface wall
energy can be reduced, and the whole magnetic energy can be made
substantially equivalent irrespective of the magnetization condition after
recording. As a result, a stable recorded condition of the memory layer
can be achieved.
| Inventors:
|
Katayama; Hiroyuki (Nara, JP);
Nakayama; Junichiro (Shiki-gun, JP);
Iketani; Naoyasu (Tenri, JP);
Ohta; Kenji (Kitakatsuragi-gun, JP)
|
| Assignee:
|
Sharp Kabushiki Kaisha (Osaka, JP)
|
| Appl. No.:
|
341310 |
| Filed:
|
November 16, 1994 |
Foreign Application Priority Data
| Current U.S. Class: |
428/212; 365/122; 369/13.46; 428/457; 428/635; 428/668; 428/686; 428/819.3; 428/900; 430/945 |
| Intern'l Class: |
G11B 011/10; G11B 013/04 |
| Field of Search: |
428/694 MM,694 EC,212,635,457,668,686,900
369/13
430/945
365/122
|
References Cited
U.S. Patent Documents
| 5278810 | Jan., 1994 | Takahashi et al. | 369/13.
|
| Foreign Patent Documents |
| 258978 | Mar., 1988 | EP.
| |
Other References
R. Malmhall et al. "Spin Directions in Exchange-Cpoupled Rare-Earth
Transition Metal Double Layer Films with In-Plane Magnetic Intermediate
Layer", Apr. 1992, Jpn.J.Appl. Phys. vol. 31, pp. 1050-1054.
T. Fukami et al. "Novel direct overwriting technology for magneto-optical
disks by exchange-coupled RE-TM quadrilayered films", May 1, 1990,
J.Appl.Phys, 67(9) pp. 4415-4416.
Y. Nakaki et al., "Overwrite Recording and Reading on Quadrilayer MO Disks
Using an Optical Head with a Red-Light LD", Journal of The Magnetics
Society of Japan, vol. 17, Supplement No. S1 (1993).
|
Primary Examiner: Resan; Stevan A.
Attorney, Agent or Firm: Conlin; David G., Oliver; Milton
Parent Case Text
This is a continuation of application Ser. No. 08/036,135 filed on Mar. 24,
1993 and now abandoned.
Claims
What is claimed is:
1. A magneto-optical recording medium, for recording by light intensity
modulation, comprising
first, second, third, and fourth magnetic layers which are laminated in
this order so that an exchange coupling exists between any adjacent layers
of said first through said fourth magnetic layers, and
a substrate supporting said first through said fourth magnetic layers,
wherein:
said first layer is a memory layer for recording information based on
magnetization direction and
said second layer is a writing layer for copying a magnetization direction
to said first magnetic layer;
respective Curie temperatures Tc.sub.i of said first through said fourth
magnetic layers satisfy an inequality
Tc.sub.3 <Tc.sub.1 <Tc.sub.2 <Tc.sub.4, where i=1, 2, 3, or 4
indicating an ordinal number of said magnetic layers; and
said second magnetic layer is formed of a material which exhibits an
in-plane magnetization at room temperature, and in which a transition
occurs from the in-plane magnetization to a perpendicular magnetization
above a transition temperature between the room temperature and the
temperature Tc.sub.3, thereby permitting overwriting by light intensity
modulation and thereby improving the stability of recording of information
on said first layer.
2. The magneto-optical recording medium as set forth in claim 1, wherein:
said third magnetic layer is a switching layer for shutting off an exchange
coupling from said fourth magnetic layer through said third magnetic layer
when a magnetization disappears from said third layer as the temperature
thereof is raised; and
said fourth magnetic layer is an initializing layer for initializing said
first magnetic layer by an exchange coupling through said third and second
magnetic layers.
3. The magneto-optical recording medium as set forth in claim 1, further
comprising
two dielectric films, transparent to laser light, provided so as to
sandwich said first to fourth magnetic layers.
4. The magneto-optical recording medium as set forth in claim 1, wherein:
said second magnetic layer is an amorphous alloy of rare-earth transition
metal.
5. The magneto-optical recording medium as set forth in claim 1, wherein
said first through fourth magnetic layers are amorphous alloys of
rare-earth transition metals.
6. The magneto-optical recording medium as set forth in claim 4, wherein:
said second magnetic layer is GdFeCo.
7. A magneto-optical recording medium for recording by light intensity
modulation, comprising
first, second, third, and fourth magnetic layers which are laminated in
this order so that an exchange coupling exists between any adjacent layers
of said first through said fourth magnetic layers, and
a substrate supporting said first through said fourth magnetic layers,
wherein:
said first layer is a memory layer for recording information based on
magnetization direction and said second layer is a writing layer for
copying a magnetization direction to said first magnetic layer;
respective Curie temperatures Tci of said first through said fourth
magnetic layers satisfy an inequality
Tc.sub.3 <Tc.sub.1 <Tc.sub.2 <Tc.sub.4, where i=1, 2, 3, or 4,
indicating an ordinal number of said magnetic layers; and
said second magnetic layer is formed of a material which exhibits an
in-plane magnetization at room temperature, and in which a transition
occurs from the in-plane magnetization to a perpendicular magnetization
above a transition temperature between the room temperature and the
temperature Tc.sub.3, thereby permitting overwriting by light intensity
modulation and thereby improving the stability of recording of information
on said first layer; and wherein said second magnetic layer is Gd.sub.x
(Fe.sub.0.82 Co.sub.0.18).sub.1-x (0.2<x<0.35) and said transition
temperature is between 60.degree. C. and 70.degree. C.
8. The magneto-optical recording medium as set forth in claim 4, wherein:
said second magnetic layer is GdCo.
9. The magneto-optical recording medium as set forth in claim 1, wherein
said magneto-optical recording medium is a magneto-optical disk for
recording by modulation of intensity of laser light projected thereon.
10. The magneto-optical recording medium as set forth in claim 1, wherein
said substrate has a property that laser light can be transmitted
therethrough.
Description
FIELD OF THE INVENTION
The present invention relates to a magneto-optical recording medium whereon
a direct overwrite is permitted by the light intensity modulation method.
BACKGROUND OF THE INVENTION
A magneto-optical recording medium whereon a direct overwrite is permitted
by the light intensity modulation method is provided with a plurality of
magnetic layers being laminated wherein an exchange coupling exists
between the adjacent magnetic layers. The above magneto-optical recording
medium has been viewed with interest for high speed data transfer, and has
been earnestly studied. For a magnetic substance of the magnetic layers,
rare-earth transition metal alloys (hereinafter referred to as RE-TM)
having a perpendicular magnetic anisotropy are known.
For the magneto-optical recording medium, a magneto-optical disk with RE-TM
quadrilayered films has been known (see J. Appl. Phys. Vol.67(1990),
FUKAMI et al. pp.4415-4416 published by American Institute of Physics).
The above magneto-optical disk does not require an initializing magnet.
This advantage can be taken to miniaturize the device. As shown in FIG. 6,
a magneto-optical disk 20 includes a transparent substrate 21 whereon a
first magnetic layer 22, a second magnetic layer 23, a third magnetic
layer 24, and a fourth magnetic layer 25 are laminated in this order.
The magneto-optical disk 20 has an exchange coupling in the adjacent
magnetic layers. Additionally, respective Curie temperatures Tci of the
magnetic layers 22, 23, 24, and 25 (i=1, 2, 3, or 4 indicating the ordinal
number of the magnetic layer) satisfy the following inequation.
Tc.sub.3 <Tc.sub.1 <Tc.sub.2, Tc.sub.4 (1)
The first magnetic layer 22 serves as a memory layer for recording thereon
information using the magnetization direction. On the other hand, neither
of the second magnetic layer 23, the third magnetic layer 24, and the
fourth magnetic layer 25 has a function as a carrier of the information.
These layers are provided so as to enable the direct overwrite by the
light intensity modulation method.
The second magnetic layer 23 is used for recording through a high
temperature process, and it is also used for initializing through a low
temperature process.
The third magnetic layer 24 serves as a switching layer for switching off
the exchange coupling from the fourth magnetic layer 25 in the high
temperature process.
The fourth magnetic layer 25 is arranged such that Tc.sub.4 thereof is set
above the temperature range in which an overwrite is permitted, so that
the magnetization direction of the fourth magnetic layer 25 is not
reversed. At room temperature, the sublattice magnetization direction of
the second magnetic layer 23 is arranged in that of the fourth magnetic
layer 25 by the exchange coupling through the third magnetic layer 24,
thereby initializing the second magnetic layer 23.
The direction of the fourth magnetic layer 25 is arranged in one direction
(for example, upward), and the Curie temperature thereof is set the
highest. Therefore, even when the temperature of the fourth magnetic layer
25 is raised by projecting the laser beam, the magnetization direction
thereof will not be reversed. Additionally, the above magnetization is the
summation of the respective sublattice magnetizations of the rare-earth
metal and the transition metal.
The overwrite process is as follows: The sublattice magnetization direction
of the first magnetic layer 22 is arranged in the initialized sublattice
magnetization direction of the fourth magnetic layer 25 by the exchange
coupling in the low temperature process. On the other hand, the sublattice
magnetization direction of the first magnetic layer 22 is arranged in the
direction opposite to the sublattice magnetization direction of the fourth
magnetic layer 25 by the external magnetic field Hex for recording (bias
magnetic field) in the high temperature process. With the above
arrangement, the overwrite is permitted on the first magnetic layer 22,
thereby recording information.
The high temperature process is a recording process wherein after arranging
the magnetization direction of the second magnetic layer 23 in the
direction of the external magnetic field Hex for recording, the arranged
magnetization direction is copied to the first magnetic layer 22 at a
temperature between Tc.sub.3 and Tc.sub.1 (see FIG. 7). Here, the
magnetization direction of the external magnetic field for recording Hex
is set the direction opposite to the sublattice magnetization direction of
the fourth magnetic layer 25.
More concretely, as shown in FIG. 7, in the case where the temperature of
the magneto-optical disk 20 is raised to the vicinity of Tc.sub.2 by
increasing the laser power, the direction of the sublattice magnetization
of the fourth magnetic layer 25 is not changed since the temperature
thereof is below Tc.sub.4. On the other hand, the magnetizations of the
first magnetic layer 22 and the third magnetic layer 24 disappear as the
temperatures thereof are respectively raised above Tc.sub.3 and Tc.sub.1.
Therefore, the magnetization of the second magnetic layer 23 is reversed in
the direction of the external magnetic field Hex for recording without
being affected by the exchange coupling from the first magnetic layer 22
nor from the third magnetic layer 24.
Thereafter, when the temperature of the magneto-optical disk 20 is dropped
below Tc.sub.1, the magnetization direction of the second magnetic layer
23 is copied to the first magnetic layer 22. Further, when the temperature
of the magneto-optical disk 20 is dropped below Tc.sub.3, the sublattice
magnetization of the third magnetic layer 24 is arranged in the direction
of the fourth magnetic layer 25 by the exchange coupling.
As the temperature of the magneto-optical disk 20 is further dropped, the
sublattice magnetization of the second magnetic layer 23 is reversed in
the direction of the fourth magnetic layer 25 by the exchange coupling
through the third magnetic layer 24. Since the coercivity of the first
magnetic layer 22 is already large in this state, the magnetization
direction thereof is not affected by the reversed magnetization of the
second magnetic layer 23.
Therefore, the sublattice magnetization direction of the first magnetic
layer 22 can be maintained in the direction opposite to the sublattice
magnetization direction of the fourth magnetic layer 25. The above
recorded state of the first magnetic layer 22 is, for example, represented
by "1" state.
On the other hand, in the low temperature process, as shown in FIG. 8, the
sublattice magnetization direction of the second magnetic layer 23
arranged in the sublattice magnetization direction of the fourth magnetic
layer 25 is copied to the first magnetic layer 22 by the exchange coupling
from the fourth magnetic layer 25 through the third magnetic layer 24.
More concretely, even when the temperature of the second magnetic layer 23
is raised to the vicinity of Tc.sub.1 by increasing the laser power
stronger than that used for reproducing, since Tc.sub.2 is set above
Tc.sub.1, and thus the coercivity thereof remains large, the sublattice
magnetization direction of the second magnetic layer 23 is not reversed by
the external magnetic field for recording.
This means that the sublattice magnetization of the second magnetic layer
23 is copied to the first magnetic layer 22 by the exchange coupling.
Thus, the sublattice magnetization direction of the first magnetic layer
22 is arranged in that of the fourth magnetic layer 25. The above state of
the first magnetic layer 22 thus initialized is represented by, for
example, "0" state.
Also in the low temperature process, even if the sublattice magnetization
of the third magnetic layer 24 disappears as the temperature thereof is
raised above Tc.sub.3, the sublattice magnetization direction thereof is
arranged in that of the fourth magnetic layer 25 by the exchange coupling
between the third and fourth magnetic layers 24 and 25 when the
temperature thereof is below Tc.sub.3.
As described, the above arrangement enables the direct overwrite by the
light intensity modulation method through the high temperature and low
temperature processes.
However, the above arrangement has the following problem. As shown in FIGS.
9(a)(b), when the recorded states "0" and "1" of the first magnetic layer
22 are compared, the respective magnetic energy conditions are not
equivalent. In the figures, a white arrow shows a magnetization, an arrow
in the white arrow shows the sublattice magnetization direction of the
transition metal or the rare-earth metal, an dotted line shows a high
interface wall energy generated between the magnetic layers.
Namely, in the above arrangement, the energy condition between the first
magnetic layer 22 and the second magnetic layer 23 is not equivalent,
since a higher energy exists in the "1" state of the first magnetic layer
22 than the "0" state of the first magnetic layer 22 by an amount of an
interface wall energy.
Therefore, the above arrangement of the magneto-optical disk 20 presents
the problem that the recorded state of the first magnetic layer 22 becomes
unstable by being reversed from the "1" state to the "0" state due to the
temperature change during reproducing or storing or due to the application
of an unexpected external magnetic field, etc. Thus, reliable recorded
states cannot be obtained.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a magneto-optical
recording medium which permits a significantly improved reliability of
recorded data.
In order to achieve the above object, the magneto-optical recording medium
in accordance with the present invention is characterized in that the
first through fourth magnetic layers, which are made of rare-earth
transition metal alloys having a perpendicular magnetic anisotropy, are
laminated in this order in such a way that an exchange coupling exists
between the adjacent magnetic layers; the magnetic layers are composed of
magnetic substances whose Curie temperatures Tci respectively satisfy an
inequation Tc.sub.3 <Tc.sub.1 <Tc.sub.2, Tc.sub.4 (i=1, 2, 3, or 4
indicating the ordinal number of the magnetic layer); and the second
magnetic layer exhibiting an in-plane magnetization at room temperature,
whereas, exhibiting a perpendicular magnetization at a temperature above a
predetermined temperature set between the room temperature and Tc.sub.3.
With the above arrangement, for example, the first magnetic layer is used
as a memory layer, and the temperature thereof is raised to the vicinity
of Tc.sub.1 by the projection of the laser beam, and is cooled off to the
room temperature. In this state, since the second magnetic layer exhibits
the perpendicular magnetization at the temperature above the predetermined
temperature, the sublattice magnetization direction of the first magnetic
layer can be arranged in the sublattice magnetization direction of the
fourth magnetic layer, thereby initializing the first magnetic layer which
is represented by, for example, the "0" state.
Namely, when the temperature is raised, magnetization remains in the third
magnetic layer until Tc.sub.3. Thus, the magnetization direction of the
fourth magnetic layer is copied to the magnetization direction of the
second magnetic layer through the third magnetic layer. Here, since the
coercivity of the first magnetic layer at the above temperature is large,
the magnetization direction of the first magnetic layer is not affected by
the magnetization direction of the second magnetic layer.
Thereafter, when the temperature is further raised to the vicinity of
Tc.sub.1, the coercivity of the first magnetic layer becomes small. As a
result, the sublattice magnetization direction of the first magnetic layer
is arranged in the sublattice magnetization direction of the second
magnetic layer, thereby initializing the first magnetic layer.
On the other hand, when the temperature is raised to the vicinity of
Tc.sub.2, the magnetization direction of the first magnetic layer can be
arranged in the direction of the external magnetic field for recording
(bias magnetic field). Therefore, by setting the direction of the external
magnetic filed opposite to the sublattice magnetization direction of the
fourth magnetic layer, the sublattice magnetization direction of the first
magnetic layer can be set opposite to the sublattice magnetization
direction of the fourth magnetic layer. In addition, the above state of
the first magnetic layer is represented by, for example, "1" state.
Namely, as the temperature is raised to the vicinity of Tc.sub.2, by the
external magnetic field for recording (bias magnetic field) second
magnetic layer is arranged in the magnetization direction of the external
magnetic field.
Thereafter, when the temperature is cooled off below Tc.sub.1, the
magnetization direction of the second magnetic layer is copied to the
first magnetic layer. Here, Tc.sub.3 of the third magnetic layer is set
substantially smaller than Tc.sub.1. When the temperature is in the range
where the coercivity of the first magnetic layer is small, the third
magnetic layer does not have the magnetization. Thus, the exchange
coupling is not exerted from the fourth magnetic layer.
Here, even if the temperature is dropped below Tc.sub.3, and the exchange
coupling of the fourth magnetic layer is exerted onto the second magnetic
layer through the third magnetic layer, as the coercivity of the first
magnetic layer is large, the effect of the magnetization direction of the
fourth magnetic layer can be avoided.
As a result, the sublattice magnetization direction of the first magnetic
layer can be reversed with respect to the sublattice magnetization
direction of the fourth magnetic layer, and the magnetization direction is
maintained. The above recorded state is represented by, for example, "1"
state.
In the above arrangement, by the difference in the application temperature,
the perpendicular magnetization directions of the first magnetic layer can
be reversed. Thus, the respective recorded states "0" state and the "1"
state can be obtained.
Furthermore, by setting the predetermined temperature higher than the room
temperature, the second magnetic layer exhibits the in-plane magnetization
at room temperature. Thus, when the first magnetic layer is in the above
recorded states, the equivalent energy condition can be achieved between
the first magnetic layer and the second magnetic layer.
Compared with the conventional model, the magneto-optical recording medium
with the above arrangement is improved by eliminating the occurrence of
the state where the high interface wall energy exists between the first
magnetic layer and the second magnetic layer which makes the energy
condition unstable.
Since the arrangement of the present invention permits the stabilization of
respective recorded states of the first magnetic layer (memory layer), the
improved reliability can be achieved with respect to the change in the
environmental condition during reproducing, or storing, such as the
temperature change, the application of an unexpected magnetic field, etc.
For a fuller understanding of the nature and advantages of the invention,
reference should be made to the ensuing detailed description taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an explanatory view showing a magnetization (white arrow) and a
sublattice magnetization (arrow) of each of the recorded magnetic layers
in the magneto-optical recording medium of the present invention.
FIG. 2 is a view showing a configuration of the magneto-optical recording
medium.
FIG. 3 is a view showing a magnetic state of the second magnetic layer in
the magneto-optical recording medium.
FIG. 4 is a graph showing the relationship between the coercivity and the
temperature of the second magnetic layer of the present embodiment.
FIG. 5 is an explanatory view showing an initialization process and a
recording process.
FIG. 6 is a view showing the configuration of the conventional
magneto-optical recording medium.
FIG. 7 is an explanatory view showing a recording process of the
magneto-optical recording medium of FIG. 6.
FIG. 8 is an explanatory view showing an initialization process of the
magneto-optical recording medium of FIG. 6.
FIG. 9 is an explanatory view showing a magnetization (white arrow) and a
sublattice magnetization (arrow) of each of the recorded magnetic layers
of the magneto-optical recording medium of FIG. 6.
DESCRIPTION OF THE EMBODIMENTS
The following description will discuss an embodiment of the present
invention with reference to FIGS. 1 through 5.
As shown in FIG. 2, a magneto-optical disk (magneto-optical recording
medium) of the present embodiment is provided with a substrate 1 made of a
material having a property that light can be transmitted therethrough such
as a glass, whereon a transparent dielectric film 2, a first magnetic
layer 3, a second magnetic layer 4, a third magnetic layer 5, a fourth
magnetic layer 6, a transparent dielectric film 7, and an overcoat film 8
are laminated in this order.
For a laminating method of the magneto-optical disk, for example, dc
sputtering method may be employed. Additionally, it is arranged such that
an exchange coupling exists between any adjacent layers of the magnetic
layers 3, 4, 5, and 6.
The second magnetic layer 4 is a ferrimagnetic material composed of
amorphous alloy of rare-earth transition metal. As shown in FIG. 3, the
selected material for the second magnetic layer 4 is such that at a
temperature below its Curie temperature, the amorphous alloy exhibits a
perpendicular magnetization in a predetermined RE content range A,
whereas, exhibits an in-plane magnetization outside the range A.
FIG. 3 shows the relationship between the rare-earth metal (RE in the
figure) content and the Curie temperature T.sub.curie in the amorphous
alloy. In raising the temperature, T.sub.A shows a temperature from which
the amorphous alloy exhibits a perpendicular magnetization, and T.sub.B
shows a temperature from which the amorphous alloy does not exhibit a
perpendicular magnetization.
As can be seen from FIG. 3, the temperature range where the second magnetic
layer 4 exhibits the perpendicular magnetization is extremely narrow.
Moreover, the perpendicular magnetization appears only in the vicinity of
a compensating composition where the magnetic moments of the rare-earth
metal and the transition metal balance with one another.
Additionally, the respective magnetic moment directions of the rare-earth
metal and the transition metal are set in an antiparallel direction. The
rare-earth metal and the transition metal have mutually different
temperature dependencies, and the magnetic moment of the transition metal
can be set greater than that of the rare-earth metal at a temperature
above the compensation temperature.
Therefore, the composition of alloy for the second magnetic layer 4 is
selected so as to satisfy the following conditions: The content of the
rare-earth metal is greater than that in the compensating composition at
room temperature. Such alloy exhibits the in-plane magnetization at room
temperature; whereas, the perpendicular magnetization appears as the
temperature is raised.
For example, when RE content P in FIG. 3 is selected, the second magnetic
layer 4 shows an abruptly rising hysteresis characteristic, i.e., the
perpendicular magnetization in the temperature range of T.sub.1 -T.sub.3.
Whereas, it exhibits the in-plane magnetization, and does not show the
hysteresis characteristic in the temperature ranges of room
temperature--T.sub.1, and T.sub.3 --T.sub.curie.
Therefore, in both low temperature and high temperature processes, when the
temperature of the second magnetic layer 4 is raised by projecting the
laser power stronger than the laser power used in reproducing, the
magnetic moment of the transition metal becomes relatively greater until
it balances with the magnetic moment of the rare-earth metal, thereby
exhibiting the perpendicular magnetization.
After the temperature of the second magnetic layer 4 is raised above
T.sub.1, and the layer 4 thus exhibits the perpendicular magnetization,
the respective sublattice magnetization directions of the magnetic layers
3-6 in both high temperature and low temperature processes can be set only
by modulating the intensity of the laser beam to be projected. Therefore,
the overwriting is permitted on the magneto-optical disk.
Additionally, the sublattice magnetization has the magnetic moment
direction of the rare-earth metal at a lattice point. Hereinafter, the
sublattice magnetization implies that the direction thereof is the
magnetic moment direction of the above rare-earth metal if not specified.
In order to enable the direct overwrite, the following materials are used
in the present embodiment.
TbFeCo (RE-TM metal alloy) is used for the first magnetic layer 3, and the
Curie temperature and thickness thereof are respectively set as follows:
Curie temperature Tc.sub.1 =180.degree.-230.degree. C.
thickness t=30-60 nm
TbFe (RE-TM metal alloy) is used for the third magnetic layer 5, and the
Curie temperature and the thickness thereof are respectively set as
follows:
Curie temperature Tc.sub.3 =100.degree.-150.degree. C.
thickness t=10-30 nm
TbCo (RE-TM metal alloy) is used for the fourth magnetic layer 6, and the
Curie temperature and thickness thereof are respectively set as follows:
Curie temperature Tc.sub.4 >250.degree. C.
thickness t=30-100 nm
When Gdx (Fe.sub.0.82 Co.sub.0.18).sub.1-x (0.2<x<0.35), is used for the
second magnetic layer 4, and the Curie temperature and thickness thereof
are respectively set as follows:
Curie temperature Tc.sub.2 =300.degree.-400.degree. C.
thickness t=10-30 nm
FIG. 4 shows a temperature dependencies of the coercivity of the second
magnetic layer 4. As is clear from the figure, when the temperature of the
second magnetic layer 4 is raised to the vicinity of
Tu=60.degree.-70.degree. C., which is lower than Tc.sub.3, the
perpendicular magnetization appears and remains until the temperature is
raised to the vicinity of 270.degree. C. Here, Tu in FIG. 4 corresponds to
T.sub.1 in FIG. 3.
The dielectric film 2 is used for magneto-optical effect enhancement which
increases the Kerr rotation angle utilizing an interference effect of the
light in the multi-layered film. As to the material for the transparent
dielectric film 2, AlN, SiN, AlSiN, etc., may be used. The thickness of
the film is set substantially the value obtained by dividing a quarter of
a reproducing wavelength by a refractive index. For example, when the
light beam with the wavelength of 800 nm is employed for reproducing, the
film thickness of the transparent dielectric film 2 is in the range of
70-100 nm.
As shown in FIGS. 5 (a)(b), the second magnetic layer 4 exhibits the
in-plane magnetization at room temperature. In addition, Tu, at which a
transition occurs on the second magnetic layer 4 from the in-plane
magnetization to the perpendicular magnetization, is required to be set
lower than Tc.sub.3. This is because in the temperature range between
T.sub.u and T.sub.c.spsb.3, the magnetization direction of the fourth
magnetic layer 6 is preferably copied to the second magnetic layer 4
through the third magnetic layer 5.
Next, the overwrite process will be described below in more detail.
First, the temperature of the predetermined small portion of the
magneto-optical disk is raised to the vicinity of Tc.sub.1, for example,
by projecting the laser beam, and then cooled off to the room temperature.
In this state, since the second magnetic layer 4 exhibits the
perpendicular magnetization at a temperature above Tu, the sublattice
magnetization direction of the first magnetic layer 3 is arranged in the
sublattice magnetization direction of the fourth magnetic layer 6, thereby
initializing the first magnetic layer 3. This recorded state is
represented by, for example, "0" state.
In raising the temperature of the third magnetic layer 5, the magnetization
remains until Tc.sub.3, and the sublattice magnetization direction of the
fourth magnetic layer 6 is copied to the sublattice magnetization
direction of the second magnetic layer 4 through the third magnetic layer
5. Here, until Tc.sub.3, since the coercivity of the first magnetic layer
3 is large, the magnetization direction of the first magnetic layer 3 is
not affected by the magnetization of the second magnetic layer 4.
As the temperature is further raised to the vicinity of Tc.sub.1, the
coercivity of the first magnetic layer 3 becomes small, and the sublattice
magnetization direction of the first magnetic layer 3 is arranged in that
of the second magnetic layer 4, thereby initializing the first magnetic
layer 3.
When the temperature is further raised to the vicinity of Tc.sub.2, the
magnetization direction of the initialized second magnetic layer 4 is
arranged by the external magnetic field Hex (bias magnetic field) for
recording whose direction is set opposite to the sublattice magnetization
direction of the fourth magnetic layer 6.
Thereafter, as the temperature is cooled off below Tc.sub.1, the sublattice
magnetization of the second magnetic layer 4 is copied to the first
magnetic layer 3. Here, Tc.sub.3 of the third magnetic layer 5 is set
substantially lower than Tc.sub.1. Thus, in the temperature range where
the coercivity of the first magnetic layer 3 is small, the magnetization
disappears from the third magnetic layer 5, and the exchange coupling
force of the fourth magnetic layer 6 is shut off.
Moreover, as the temperature further drops below Tc.sub.3, even if the
exchange coupling of the fourth magnetic layer 6 is exerted onto the
second magnetic layer 4 through the third magnetic layer 5, since the
coercivity of the first magnetic layer 3 becomes large, the effect from
the fourth magnetic layer 6 can be avoided.
In this way, the sublattice magnetization direction of the first magnetic
layer 3 can be reversed with respect to the sublattice magnetization
direction of the fourth magnetic layer 6, and the reversed magnetization
can be maintained. This recorded state is for example represented by "1"
state.
With the above arrangement of the magneto-optical disk, since the
perpendicular magnetization directions of the first magnetic layer 3 can
be reversed by varying the application temperature, the respective
recorded states, i.e., "1" state and "0" state can be obtained.
Furthermore, by setting Tu higher than the room temperature, the second
magnetic layer 4 exhibits the in-plane magnetization at room temperature.
This permits an equivalent energy condition between the first magnetic
layer 3 and the second magnetic layer 4 irrespective of the above recorded
states of the first magnetic layer 3.
Namely, the magneto-optical disk is arranged such that the respective
magnetization directions of magnetic layers 3-6 at room temperature which
correspond to the recorded states of "0" state and "1" state are as shown
in FIGS. 1(a) and (b).
Therefore, when compared with the conventional model, the magneto-optical
disk is significantly improved by eliminating the occurrence of the state
where the high interface wall energy exists between the first magnetic
layer 3 and the second magnetic layer 4, and the sublattice magnetization
directions of the respective recorded states are antiparallel. As a
result, the equivalent energy conditions can be achieved between the above
recorded states, and the interface wall energy can be reduced.
Therefore, with the above arrangement, unstable energy condition between
the first magnetic layer 3 and the second magnetic layer 4 of the
magneto-optical disk due to the high interface wall energy can be
prevented.
Since the respective recorded conditions of the first magnetic layer 3
(memory layer) can be stabilized, the significantly improved reliability
can be achieved with respect to the change in the reproducing or storing
condition, etc.
Additionally, the second magnetic layer 4 is not limited to GdFeCo, and
GdCo which has a similar property may be used as well. For the first
magnetic layer 3, DyFeCo, TbDyCo, GdDyFeCo, or GdTbFeCo may be used, and
for the third magnetic layer 5, GdFe, or DyFeCo may be used. Similarly,
for the fourth magnetic layer 6, GdTbCo may be used as well.
Although the magneto-optical disk is employed as the magneto-optical
recording medium in the present embodiment, the present invention is not
limited to this, and a magneto-optical tape, a magneto-optical card, etc.,
may be used as well.
Similarly, the sublattice magnetization direction is set the magnetic
moment direction of the rare-earth metal at lattice point. However,
depending on the composition of the alloys of the magnetic layers, i.e.,
RErich or TMrich, the magnetic moment direction of the transition metal at
a lattice point may be used as well.
The invention being thus described, it will be obvious that the same may be
varied in many ways. Such variations are not to be regarded as a departure
from the spirit and scope of the invention, and all such modifications as
would be obvious to one skilled in the art are intended to be included
within the scope of the following claims.
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